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Thermal neutron analysis (TNA) explosive detection based on electronic neutron generators
Nuclear Instruments and Methods in Physics Research B 99 (1995) 739-742
Beam Interactions with Materials A Atoms
ELSEVIER
Thermal neutron analysis (TNA) explosive detection based on electronic neutron generators W.C. Lee *, D.B. Mahood, P. Ryge, P. Shea, T. Gozani Science Applications International Corporation, 2950 Patrick Henry Dr., Santa Clara, CA 95054, USA
Abstract Thermal neutron analysis explosive detection systems have been developed and demonstrated for inspection of checked airline baggage and for detection of buried land mines. Thermal neutrons from a moderated neutron source impinge on the inspected object, and the resulting capture gamma ray signatures provide detection information. Isotopic neutron sources, e.g. ‘s’Cf, are compact, economical and reliable, but they are subject to the licensing requirements, safety concerns and public perception problems associated with radioactive material. These are mitigated by use of an electronic neutron generator - an ion accelerator with a target producing neutrons by a nuclear reaction such as D(d, nj3He or ‘Be(d, n)‘“B. With suitable moderator designs based on neutron transport codes, operational explosive detection systems can be built and would provide effective alternatives to radioactive neutron sources. Calculations as well as laboratory and field experience with three generator types will be presented.
1. Introduction Under FAA sponsorship, thermal neutron analysis (TNA) explosive detection systems (EDS) for inspection of checked airline baggage have been developed and demonstrated. Two parallel development paths were initiated, one using a *‘*Cf radioisotope source and one using an electronic neutron generator (ENG) consisting of a small particle accelerator with a neutron production target. A second generation development produced the californium-based TNA which had been deployed in various international airports. This operation has yielded a wealth of information including data from 1.1 million passenger bags [l]. The TNA system, as previously described [2], is based on a well-known theory of thermal neutron activation. This technique is among many available nuclear techniques [3,4]. Fast neutrons, generated from either 252Cf or an ENG, are slowed down to very low energy, approximately 0.025 eV. When these low energy or thermai neutrons interact with the bag contents, characteristic yrays are given off. The y-rays of interest are 10.83 MeV and 2.223 MeV lines from nitrogen and hydrogen respectively. An array of y-ray detectors record the signatures and reconstruct the elemental density image. Each of the neutron sources has its advantages over the other. The “‘Cf source is more compact, economical and
* Corresponding author. Tel. + 1 408 727 0607, fax + 1 408 727 8748. 0168-583X/95/$09.50
0 1995 Elsevier
S.SDI 0168-583X(95)00221-9
absolutely reliable since neutrons are emitted constantly (whether desired or not). A neutron generator is more bulky, expensive, and, like any equipment, not totally reliable. However, since it can be turned off and thereby cease emitting radiation, it was considered inherently safer and expected to be less difficult in terms of regulation and licensing.
2. Neutron production Various deuteron beam reactions can be employed for producing neutrons. Neutron production yields are shown in Fig. 1 for various reactions [5]. The neutron production cross sections for the deutron beam reactions are compiled by Liskien and Paulsen [6]. The prolific T(d, nj4He reaction is not suitable for TNA application because the 14.7 MeV neutrons are too difficult to shield and produce excessive background. The D(d, nJ3He reaction has a high yield with low particle energies. The energy distribution of the neutrons emitted from the D(d, nj3He reaction at low deuteron beam energy is almost monoenergetic, resulting in a good neutron thermalization efficiency. The ‘Be(d, n)“B reaction can also be employed for the production of neutrons. Its neutron energy distribution summed over angles is shown in Fig. 2 [9]. The major advantage of the D(d, nj3He reaction is that high neutron yields can be achieved at low voltage or beam energy but requires high beam current. The monoenergetic output results in more efficient thermal neutron
Science B.V. All rights reserved XV. EXPLOSIVES/CONTRABAND DETECTION
W.C. Lee et al. /Nucl. Instr. and Meth. in Phys. Res. B 99 (1995) 739-742
740
C’*(dn)N”
(THICK)
‘(THICK)
T (d nl He’ b3mg/cm’l T-Ti
DldnJHc’
m5mg/cm’l D- Ti
5
Ii
2
IO'
0.1
2
iii
5 1.0 2 5 IO z BOMBARDING ENERGY b-&v)
il 5
100
Fig. 1. Neutron yields - (d, n) and (a, N) reactions.
production. The principal disadvantage is that the target is short lived due to deuterium sputtering erosion which occurs at the high beam currents needed to supply adequate neutron yield. The ‘Be(d, n)“B reaction has the advantage of producing high neutron output but requires higher beam energy. The major advantage of the reaction is that solid beryllium neutron production targets can be employed and will last indefinitely. A disadvantage is the production of gamma rays along with the broad neutron spectrum resulting in a higher background.
is ionized and focused into a beam. The beam is produced at a potential of 150-200 kV, relative to the target which is at ground potential. The high deuteron current resulted in limited tube lifetime due to the high power loading on the target. Explosive detection performance of the EDS with the Kaman generator was tested at San Francisco and Los Angeles airports on passenger baggage and at San Francisco on air cargo. Probability of detection, probability of false alarm and throughput goals were met. The useful life of the tubes varied considerably but was generally about 250 hours of continuous full output operation. The National Electrostatic Corporation (NEC) Model 3SH accelerator using the ‘Be(d, n)“B reaction was installed and tested in the EDS. The Model 3SH is contained in a sealed tank filled with SF, to insulate the ion source, accelerator column, and pellet-chain high voltage charging system. The NEC accelerator was rated at a maximum beam current of 20 &A at 1 MeV energy with neutron outputs as high as 1 X lo9 n/s. Operation of the accelerator was fairly simple. The stability of the neutron output was excellent. Other than minor adjustments of the corona probe used for voltage regulation, the NEC accelerator has been operated continuously with no major faults or maintenance required for a period of 10000 hours. The NEC Pelletron has proven to be a reliable, safe, and economical neutron generator. A neutron generator made by Science Research Laboratory (SRL) was evaluated in detecting buried mines in a laboratory setup. The SRL neutron generator was also based on the ‘Be(d, n)“B reaction. The positively charged deuterium ions from an RF ion source was accelerated
3. Neutron generator systems The Kaman A711 Neutron Generator was integrated with the prototype EDS. The Kaman generator, using the D(d, n)3He reaction, produced a 180 keV deuteron beam with currents of up to 4.0 mA which resulted in a total neutron output of 5 X lo* n/s at an energy of about 2.6 MeV. The compact accelerator head consists of an SF, insulated pressure vessel containing the glass and metal sealed accelerator tube, comprised of a deuterium ion source, acceleration electrodes and titanium deuteride neutron production target. At one end of the sealed tube, deuterium gas is released by heating a “getter” material. It
300
150
0 0
I
2 NEUTRON
3 ENERGY
4 (Mev) DATA
5
FROM
6
Sh57
Fig. 2. Neutron spectra from the ‘Be(d, n)‘*B reaction energy.
- low
W.C. Lee et al. /Nucl.
I
Fig. 3. Gamma-ray spectra of sand with and without SRL neutron generator.
741
Instr. and Meth. in Phys. Res. B 99 (1995) 739-742
mine from
the detector counts measured by the system. The distance between the centers of the two clusters (with and without explosives) in feature space can be used to predict the performance and compare different EDS systems. To compare between systems on a common ground, the same set of ten bags are used repeatedly, and the five best features are deduced by the stepwise discriminant procedure. Since the ten bags are chosen to represent the extremes of the distribution of passenger bags, there is a high correlation of the distance between the two clusters as measured by the ten bags with the detection and the false alarm rates measured on the larger set of passenger bags. The separation is calculated from the means for the two clusters in units of the standard deviation of the points about the mean. This measure of separation, afso known as the Mahalanobis distance D, is computed by SAS [8] procedure DISCRIM. Its formula is as follows: D’=(X,-Xj)*COV-‘(Xi-X,),
a compact 600 kV accelerator to a beryllium target. The power was supplied by a rugged cascade voltage multiplier which has higher current capability and lower sensitivity to mechanical vibration than a electrostatic generator. This scheme was more appropriate for the mobile mine detection system since the neutron generator is expected to endure through many vibrations and bumps. The laboratory measurements pushed this accelerator to 500 kV and 100 p.A, generating maximum neutron output to 2.4 X lo* per second. Despite its low neutron yield, the SRL generator was the smallest of the three neutron generators and was easy to handle. The neutron moderator design was slightly different from the others since the application here was to detect mines buried under ground, which is more dense than passenger suitcases. Therefore, the neutron energy spectrum was typically hard, allowing the ground to do the majority of thermalization. Fig. 3 shows a detector response from sand with and without a mine. through
4. Explosive detection performance based EDS
where the i and j refer to two clusters, X is the mean vector for the cluster, and COV ’ is the inverse of the covariance matrix between the clusters. The calculated Mahalanobis distance showed that the performance of the NEC accelerator based EDS was comparable to that of the californium-based TNA units. One of the advantages of an electronic neutron generator is its ability to produce variable neutron output by the adjustment of its beam current. This unique feature was exploited. and the detector performances were compared at three different beam currents, namely 2 ALA,5 kA, and 10 ~.LA.The detector performance is measured in terms of its figure of merit (FOM), which is defined as
where S represents count rate due to the source alone without background and B represents the count rate due to background. The background count rate was measured
of NEC accelerator
A large number of bags must be run through the system in order to establish its performance with statistical validity. This process is very time consuming. Therefore, a simpler technique for estimating the performance from a smaller number of measurements was desired. This simpler technique involves measuring ten standard bags with and without explosives. For convenience reasons, explosive simulants were used in place of actual explosives. The bags were chosen to cover the full range, as measured by the nitrogen content and flux absorption, of actual suitcases encountered in airports. The EDS computer makes explosive detection decisions by discriminant analysis [7] using features which are a mathematical combination of
Fig. 4. Gamma-ray values.
spectra from NEC-EDS
at three beam current
XV. EXPLOSIVES/CONTRABAND
DETECTION
742
W.C. Lee et al. /Nucl.
Instr. and Meth. in Phys. Res. B 99 (1995) 739-742
with a non-nitrogeneous sample, while the net signal was determined by subtracting the background from the gross count rate with a nitrogeneous sample. The FOM measures the ratio of signal to statistical background in terms of the number of (counting statistics) standard deviations. This measure is especially important in EDS because of its short measurement time. The signal of interest, is the 10.83 MeV -y-ray from the thermal neutron capture in nitrogen. Fig. 4 shows the pulse height spectrum acquired at the three beam currents and their respective FOMs. It is evident that the FOM increases with neutron output in this range. This emphasizes the important advantage of using an ENG, namely the ability to vary neutron output.
5. Conclusion The studies have shown that an operational airport baggage inspection system or a land mine detection system based on an electronic neutron generator can be built and would be a practical and effective alternative to a system employing radioactive materials. However, the neutron generators are sometimes unreliable and bulky. It is the technical challenge to build better and more reliable accelerators.
Acknowledgements Authors would like to thank FAA and ARPA for sponsoring the programs in which various neutron generators were evaluated.
References [l] W. Lee, J. Bendahan, P. Shea and V. Leung, Nucl. Instr. and Meth. A 353 (1994) 641. [2] P. Shea, T. Gozani and H. Bozorgmanesh, Nucl. Instr. and Meth. 299 (1990) 444. [3] T. Gozani, Principles of nuclear-based explosive detection systems, Proc. 1st Int. Symp. on Explosive Detection Technology, 1991, p. 27. [4] L. Grodzins, Nucl. Instr. and Meth. B 56/57 (1991) 829. [5] A.E. Burrill, Neutron Production and Protection, High Voltage Engineering Corporation, Burlington, MA. [6] H. Liskien and A. Paulsen, Nucl. Data Tables (1973) 569. [7] T.W. Anderson, Introduction to Multivariate Statistical Analysis (Wiley, New York, 1958). [8] SAS User’s Guide: Statistics Version 5, SAS Institute Inc., Cary, NC (1985). [9] AI. Shpetni, Sov. Phys. JETP 5 (1957) 357.
Beam Interactions with Materials A Atoms
ELSEVIER
Thermal neutron analysis (TNA) explosive detection based on electronic neutron generators W.C. Lee *, D.B. Mahood, P. Ryge, P. Shea, T. Gozani Science Applications International Corporation, 2950 Patrick Henry Dr., Santa Clara, CA 95054, USA
Abstract Thermal neutron analysis explosive detection systems have been developed and demonstrated for inspection of checked airline baggage and for detection of buried land mines. Thermal neutrons from a moderated neutron source impinge on the inspected object, and the resulting capture gamma ray signatures provide detection information. Isotopic neutron sources, e.g. ‘s’Cf, are compact, economical and reliable, but they are subject to the licensing requirements, safety concerns and public perception problems associated with radioactive material. These are mitigated by use of an electronic neutron generator - an ion accelerator with a target producing neutrons by a nuclear reaction such as D(d, nj3He or ‘Be(d, n)‘“B. With suitable moderator designs based on neutron transport codes, operational explosive detection systems can be built and would provide effective alternatives to radioactive neutron sources. Calculations as well as laboratory and field experience with three generator types will be presented.
1. Introduction Under FAA sponsorship, thermal neutron analysis (TNA) explosive detection systems (EDS) for inspection of checked airline baggage have been developed and demonstrated. Two parallel development paths were initiated, one using a *‘*Cf radioisotope source and one using an electronic neutron generator (ENG) consisting of a small particle accelerator with a neutron production target. A second generation development produced the californium-based TNA which had been deployed in various international airports. This operation has yielded a wealth of information including data from 1.1 million passenger bags [l]. The TNA system, as previously described [2], is based on a well-known theory of thermal neutron activation. This technique is among many available nuclear techniques [3,4]. Fast neutrons, generated from either 252Cf or an ENG, are slowed down to very low energy, approximately 0.025 eV. When these low energy or thermai neutrons interact with the bag contents, characteristic yrays are given off. The y-rays of interest are 10.83 MeV and 2.223 MeV lines from nitrogen and hydrogen respectively. An array of y-ray detectors record the signatures and reconstruct the elemental density image. Each of the neutron sources has its advantages over the other. The “‘Cf source is more compact, economical and
* Corresponding author. Tel. + 1 408 727 0607, fax + 1 408 727 8748. 0168-583X/95/$09.50
0 1995 Elsevier
S.SDI 0168-583X(95)00221-9
absolutely reliable since neutrons are emitted constantly (whether desired or not). A neutron generator is more bulky, expensive, and, like any equipment, not totally reliable. However, since it can be turned off and thereby cease emitting radiation, it was considered inherently safer and expected to be less difficult in terms of regulation and licensing.
2. Neutron production Various deuteron beam reactions can be employed for producing neutrons. Neutron production yields are shown in Fig. 1 for various reactions [5]. The neutron production cross sections for the deutron beam reactions are compiled by Liskien and Paulsen [6]. The prolific T(d, nj4He reaction is not suitable for TNA application because the 14.7 MeV neutrons are too difficult to shield and produce excessive background. The D(d, nJ3He reaction has a high yield with low particle energies. The energy distribution of the neutrons emitted from the D(d, nj3He reaction at low deuteron beam energy is almost monoenergetic, resulting in a good neutron thermalization efficiency. The ‘Be(d, n)“B reaction can also be employed for the production of neutrons. Its neutron energy distribution summed over angles is shown in Fig. 2 [9]. The major advantage of the D(d, nj3He reaction is that high neutron yields can be achieved at low voltage or beam energy but requires high beam current. The monoenergetic output results in more efficient thermal neutron
Science B.V. All rights reserved XV. EXPLOSIVES/CONTRABAND DETECTION
W.C. Lee et al. /Nucl. Instr. and Meth. in Phys. Res. B 99 (1995) 739-742
740
C’*(dn)N”
(THICK)
‘(THICK)
T (d nl He’ b3mg/cm’l T-Ti
DldnJHc’
m5mg/cm’l D- Ti
5
Ii
2
IO'
0.1
2
iii
5 1.0 2 5 IO z BOMBARDING ENERGY b-&v)
il 5
100
Fig. 1. Neutron yields - (d, n) and (a, N) reactions.
production. The principal disadvantage is that the target is short lived due to deuterium sputtering erosion which occurs at the high beam currents needed to supply adequate neutron yield. The ‘Be(d, n)“B reaction has the advantage of producing high neutron output but requires higher beam energy. The major advantage of the reaction is that solid beryllium neutron production targets can be employed and will last indefinitely. A disadvantage is the production of gamma rays along with the broad neutron spectrum resulting in a higher background.
is ionized and focused into a beam. The beam is produced at a potential of 150-200 kV, relative to the target which is at ground potential. The high deuteron current resulted in limited tube lifetime due to the high power loading on the target. Explosive detection performance of the EDS with the Kaman generator was tested at San Francisco and Los Angeles airports on passenger baggage and at San Francisco on air cargo. Probability of detection, probability of false alarm and throughput goals were met. The useful life of the tubes varied considerably but was generally about 250 hours of continuous full output operation. The National Electrostatic Corporation (NEC) Model 3SH accelerator using the ‘Be(d, n)“B reaction was installed and tested in the EDS. The Model 3SH is contained in a sealed tank filled with SF, to insulate the ion source, accelerator column, and pellet-chain high voltage charging system. The NEC accelerator was rated at a maximum beam current of 20 &A at 1 MeV energy with neutron outputs as high as 1 X lo9 n/s. Operation of the accelerator was fairly simple. The stability of the neutron output was excellent. Other than minor adjustments of the corona probe used for voltage regulation, the NEC accelerator has been operated continuously with no major faults or maintenance required for a period of 10000 hours. The NEC Pelletron has proven to be a reliable, safe, and economical neutron generator. A neutron generator made by Science Research Laboratory (SRL) was evaluated in detecting buried mines in a laboratory setup. The SRL neutron generator was also based on the ‘Be(d, n)“B reaction. The positively charged deuterium ions from an RF ion source was accelerated
3. Neutron generator systems The Kaman A711 Neutron Generator was integrated with the prototype EDS. The Kaman generator, using the D(d, n)3He reaction, produced a 180 keV deuteron beam with currents of up to 4.0 mA which resulted in a total neutron output of 5 X lo* n/s at an energy of about 2.6 MeV. The compact accelerator head consists of an SF, insulated pressure vessel containing the glass and metal sealed accelerator tube, comprised of a deuterium ion source, acceleration electrodes and titanium deuteride neutron production target. At one end of the sealed tube, deuterium gas is released by heating a “getter” material. It
300
150
0 0
I
2 NEUTRON
3 ENERGY
4 (Mev) DATA
5
FROM
6
Sh57
Fig. 2. Neutron spectra from the ‘Be(d, n)‘*B reaction energy.
- low
W.C. Lee et al. /Nucl.
I
Fig. 3. Gamma-ray spectra of sand with and without SRL neutron generator.
741
Instr. and Meth. in Phys. Res. B 99 (1995) 739-742
mine from
the detector counts measured by the system. The distance between the centers of the two clusters (with and without explosives) in feature space can be used to predict the performance and compare different EDS systems. To compare between systems on a common ground, the same set of ten bags are used repeatedly, and the five best features are deduced by the stepwise discriminant procedure. Since the ten bags are chosen to represent the extremes of the distribution of passenger bags, there is a high correlation of the distance between the two clusters as measured by the ten bags with the detection and the false alarm rates measured on the larger set of passenger bags. The separation is calculated from the means for the two clusters in units of the standard deviation of the points about the mean. This measure of separation, afso known as the Mahalanobis distance D, is computed by SAS [8] procedure DISCRIM. Its formula is as follows: D’=(X,-Xj)*COV-‘(Xi-X,),
a compact 600 kV accelerator to a beryllium target. The power was supplied by a rugged cascade voltage multiplier which has higher current capability and lower sensitivity to mechanical vibration than a electrostatic generator. This scheme was more appropriate for the mobile mine detection system since the neutron generator is expected to endure through many vibrations and bumps. The laboratory measurements pushed this accelerator to 500 kV and 100 p.A, generating maximum neutron output to 2.4 X lo* per second. Despite its low neutron yield, the SRL generator was the smallest of the three neutron generators and was easy to handle. The neutron moderator design was slightly different from the others since the application here was to detect mines buried under ground, which is more dense than passenger suitcases. Therefore, the neutron energy spectrum was typically hard, allowing the ground to do the majority of thermalization. Fig. 3 shows a detector response from sand with and without a mine. through
4. Explosive detection performance based EDS
where the i and j refer to two clusters, X is the mean vector for the cluster, and COV ’ is the inverse of the covariance matrix between the clusters. The calculated Mahalanobis distance showed that the performance of the NEC accelerator based EDS was comparable to that of the californium-based TNA units. One of the advantages of an electronic neutron generator is its ability to produce variable neutron output by the adjustment of its beam current. This unique feature was exploited. and the detector performances were compared at three different beam currents, namely 2 ALA,5 kA, and 10 ~.LA.The detector performance is measured in terms of its figure of merit (FOM), which is defined as
where S represents count rate due to the source alone without background and B represents the count rate due to background. The background count rate was measured
of NEC accelerator
A large number of bags must be run through the system in order to establish its performance with statistical validity. This process is very time consuming. Therefore, a simpler technique for estimating the performance from a smaller number of measurements was desired. This simpler technique involves measuring ten standard bags with and without explosives. For convenience reasons, explosive simulants were used in place of actual explosives. The bags were chosen to cover the full range, as measured by the nitrogen content and flux absorption, of actual suitcases encountered in airports. The EDS computer makes explosive detection decisions by discriminant analysis [7] using features which are a mathematical combination of
Fig. 4. Gamma-ray values.
spectra from NEC-EDS
at three beam current
XV. EXPLOSIVES/CONTRABAND
DETECTION
742
W.C. Lee et al. /Nucl.
Instr. and Meth. in Phys. Res. B 99 (1995) 739-742
with a non-nitrogeneous sample, while the net signal was determined by subtracting the background from the gross count rate with a nitrogeneous sample. The FOM measures the ratio of signal to statistical background in terms of the number of (counting statistics) standard deviations. This measure is especially important in EDS because of its short measurement time. The signal of interest, is the 10.83 MeV -y-ray from the thermal neutron capture in nitrogen. Fig. 4 shows the pulse height spectrum acquired at the three beam currents and their respective FOMs. It is evident that the FOM increases with neutron output in this range. This emphasizes the important advantage of using an ENG, namely the ability to vary neutron output.
5. Conclusion The studies have shown that an operational airport baggage inspection system or a land mine detection system based on an electronic neutron generator can be built and would be a practical and effective alternative to a system employing radioactive materials. However, the neutron generators are sometimes unreliable and bulky. It is the technical challenge to build better and more reliable accelerators.
Acknowledgements Authors would like to thank FAA and ARPA for sponsoring the programs in which various neutron generators were evaluated.
References [l] W. Lee, J. Bendahan, P. Shea and V. Leung, Nucl. Instr. and Meth. A 353 (1994) 641. [2] P. Shea, T. Gozani and H. Bozorgmanesh, Nucl. Instr. and Meth. 299 (1990) 444. [3] T. Gozani, Principles of nuclear-based explosive detection systems, Proc. 1st Int. Symp. on Explosive Detection Technology, 1991, p. 27. [4] L. Grodzins, Nucl. Instr. and Meth. B 56/57 (1991) 829. [5] A.E. Burrill, Neutron Production and Protection, High Voltage Engineering Corporation, Burlington, MA. [6] H. Liskien and A. Paulsen, Nucl. Data Tables (1973) 569. [7] T.W. Anderson, Introduction to Multivariate Statistical Analysis (Wiley, New York, 1958). [8] SAS User’s Guide: Statistics Version 5, SAS Institute Inc., Cary, NC (1985). [9] AI. Shpetni, Sov. Phys. JETP 5 (1957) 357.